As high-quality broadband Internet access services using optical fibers are disseminated, services to integrate a traditional telephone service into an Internet access service have been increasing. Such integration services realize a telephone service using Internet access lines without using telephone lines connected into subscribers' homes. A telephone service can be integrated into an Internet access network by, broadly classified, the following two methods:
(1) Configuring softswitch using SIP (RFC3261: Session Initiation Protocol) and H.248/Megaco (RFC3525: Gateway Control Protocol). No telephone network is used.
(2) Connecting an Internet access network to an existing telephone network using a voice gateway which has a signaling function such as an SIP function or an H.248/Megaco on the Internet access network side and a signaling function for traditional telephone switching on the telephone network side and which can convert voice data bidirectionally between a packet format and a TDM (time division multiplexed) format.
The method (1) requiring no telephone switch is economically advantageous. In terms of reliability, however, services realized by the method (1) are inferior to those offered by a traditional telephone network. Furthermore, there can be cases where an Internet access network realized by the method (1) cannot support some supplementary services (such as three-way talk and emergency call services) offered by a traditional telephone network. The method (2) is disadvantageous in that it requires an existing telephone network to be retained. However, since it connects an Internet access network directly to an existing telephone network, it can realize telephone services which are comparable in quality to traditional telephone services and which can support supplementary services equivalent to those offered by a traditional telephone network. Another advantage of the method (2) is that it allows telephone companies which have been providing traditional telephone services to make effective use of their existing equipment.
The present invention relates to a voice gateway which, in a network realized by the method (2), connects an Internet access network and an existing telephone network, and an OLT (optical line terminator) and an ONT (optical line terminal) which provide Internet access lines of optical fibers.
Many voice gateways are available for use in the method (2) to connect an Internet network (IP network) and a telephone network, but they are intended for application to:
(A) A network operating format in which an access network portion of an existing telephone network is utilized while a backbone network portion of the existing telephone network is replaced with an IP network to reduce transit line cost.
In terms of a voice gateway intended for application to the network operating format (A), a technique which takes into consideration how to cope with a failure of a general IP network with low reliability is disclosed in JP-A No. 2002-290551. The technique is aimed at keeping a call active even when the relevant IP network fails or even when a server installed in the IP network, for example, an SIP server or a Media Gateway Controller (MGC) based on H.248/Megaco develops a failure.